Lesson focus: thermal decompositionBreaking down a substance into simpler substances by heating it. uses energy transferred by heating to break one compound into simpler products.
Recognise thermal decomposition, construct equations and interpret evidence from carbonateA compound containing the carbonate group (CO₃), such as calcium carbonate (chalk), that gives off carbon dioxide when it reacts with acid. heating.
Build on earlier learning
Burning fuelsCombustion releases energy overall; thermal decomposition requires sustained heating.
Practical
Heating carbonates
- Heat a small amount of copper carbonate in a test tube.
- Bubble the gas through limewater.
- Note the colour change of the solid and whether limewater turns cloudy.
Record: Colour before and after, and the gas produced.
Safety: Remove the delivery tube from the limewater before stopping heating to prevent suck-back.
You will be successful when you can…
- Define thermal decomposition.
- Identify one reactant and multiple products.
- Write carbonate decomposition equations.
- Use a gas test as product evidence.
Words that carry the science
- thermal decomposition
- Breakdown of a compound by heating.
- carbonate
- A compound containing the carbonate ion.
- oxide
- A compound containing oxygen bonded to another element.
- limewater
- A solution used to test for carbon dioxide.
- decomposition
- A reaction in which one reactant forms two or more products.
One compound forms simpler products
01One compound forms simpler products
In thermal decompositionBreaking down a substance into simpler substances by heating it., energy transferred by heating breaks down a compound. The pattern is one reactant → two or more products.
Many metal carbonateA compound containing the carbonate group (CO₃), such as calcium carbonate (chalk), that gives off carbon dioxide when it reacts with acid. decompose to a metal oxideA compound made when an element joins with oxygen, such as rust (iron oxide). and carbon dioxide, but temperature requirements vary and some carbonates are more thermally stable.
02Limestone links laboratory and industry
Calcium carbonate → calcium oxide + carbon dioxide. Limestone is mainly calcium carbonate, while quicklime is calcium oxide.
Industrial lime manufacture needs high temperatures and releases carbon dioxide from fuel use and from the carbonate reaction.
03Product tests support the equation
Carbon dioxide turns limewaterA clear liquid that turns cloudy (milky) when carbon dioxide is bubbled through it. cloudy because a fine calcium carbonate precipitate forms.
A mass decrease in an open heated tube can support gas loss, but conservation still applies when the escaping gas and surroundings are included.
Thermal decomposition of copper carbonate
Thermal decomposition is when heat breaks a substance down into simpler substances. Green copper carbonate breaks down into black copper oxide and carbon dioxide gas. The gas escapes, so the mass in the tube goes down.
Controls
copper carbonate → copper oxide + carbon dioxide (CuCO₃ → CuO + CO₂). The final masses come from relative formula masses; the heating time is illustrative.
Interpret copper carbonate heating
Green copper carbonate becomes black and a gas clouds limewater.
- A colour change supports formation of a different solid.
- Cloudy limewater identifies carbon dioxide.
- The remaining black solid is copper oxide.
- Write copper carbonate → copper oxide + carbon dioxide.
How to make the reasoning visible: Two product observations support the thermal-decomposition equation.
Carbonate decomposition
Use the evidence from four heating trials.
| Reactant | Solid before | Solid after | Gas test |
|---|---|---|---|
| copper carbonate | green | black | limewater cloudy |
| calcium carbonate | white | white | CO₂ after strong heat |
| zinc carbonate | white | yellow hot/white cold | limewater cloudy |
| sodium carbonate | white | no observed change | negative |
Which trials support decomposition?
Why is “no observed change” not proof that decomposition is impossible?
Check your understanding
Answer from memory, then check against the suggested answer.
Define thermal decomposition.
General metal carbonate products?
Positive CO₂ test?
Write calcium carbonate decomposition as a word equation.
Why can the measured solid mass fall while total mass is conserved?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Copper carbonate is heated in an open test tube. The mass of the tube and its contents goes down.
Name the type of reaction and the two products.
(3)Explain why the mass goes down, even though mass is conserved.
(2)Total for this question: 5 marks
Command word: Explain both the chemistry and the measurement boundary.